Considerable interest has existed in the copolymerization of acrylates with norbornenes because of the potential benefits of combining the useful properties of the homopolymers of the two monomers. For example, polyacrylates are valued for their extreme hardness and adhesive properties, and are used to form clear, glass-like materials such as Lucite™ and Plexiglas™. In addition, polymers formed using acrylate-type monomers have been found useful in microelectronic fabrication, for example, in advanced photoresist compositions where high-resolution images are desired. Polynorbornenes, on the other hand, are capable of resisting high temperatures and, thus, typically are employed in applications that necessitate high-temperature stability. In addition, polymers formed using norbornene-type monomers have been found useful in microelectronic fabrication, for example, in advanced photoresist compositions where high selectivity during plasma etching processes is desired.
Polymerization of acrylic derivatives is disclosed, for example, in U.S. Pat. No. 4,849,488. In that patent, certain phosphorus compounds are used as polymerization catalysts. Preferred acrylic derivatives that are polymerized in accordance with that patent are acrylates and methacrylates of monovalent and polyvalent alcohols. Copolymers of at least two different acrylic derivatives are also disclosed. There is no disclosure in that patent relevant to the possibility of copolymerizing an acrylic derivative with norbornene or a norbornene derivative or the use of such a copolymer in a photoresist composition.
Some attempts to copolymerize acrylates with norbornenes are known but have met with only modest success. Such modest success has been thought to be a result of a disparity in the mechanisms by which the respective monomers polymerize. U.S. Pat. No. 3,697,490, for example, discloses copolymers of alkoxy alkyl acrylates, alkyl acrylates and substituted norbornene, wherein the copolymers contain only about 0.5 to about 5.5 percent by weight substituted norbornene compounds. The copolymers described in that patent typically are prepared in batch reactions, or the monomers may be proportioned to a reactor containing water and other desired polymerization additives. The patent indicates that best results are generally obtained at polymerization temperatures in the range of 5° C. to 50° C. in the presence of water containing a free radical generating catalyst and surface active agents. There is no disclosure in the '490 patent relevant to the use of the described copolymers for photoresist compositions.
U.S. Pat. No. 6,034,259 broadly discloses a process for polymerizing ethylene, acyclic olefins, and/or selected cyclic olefins, and optionally selected olefinic esters or carboxylic acids. The polymerization reactions are catalyzed by selected transition metal compounds, and sometimes other co-catalysts. Also described in that patent is the synthesis of linear alpha-olefins by the oligomerization of ethylene using as a catalyst system a combination of a nickel compound having a selected alpha-diimine ligand and a selected Lewis or Bronsted acid, or by contacting selected alpha-diimine complexes with ethylene. The '259 patent also discloses polymerization of substituted norbornenes using the disclosed alpha-diimine-containing catalysts systems. However, such patent does not disclose or suggest the use of the described copolymers for photoresist compositions.
Japanese publication JP040063810 relates to copolymers that contain units derived from three essential monomers, namely: (i) an acrylic ester and/or methacrylic ester monomer, (ii) a monomer that possesses a norbornene skeleton, and (iii) a monomer that can be radical-polymerized with the monomer that contains the norbornene skeleton. The copolymers must contain from about 0.5 to about 35 mole % of units derived from radical-polymerizable monomer (iii). The compounds that are disclosed as being suitable for use as the monomer (iii) include esters of maleic acid and an aliphatic alcohol which contains from 1 to 12 carbon atoms, maleic anhydride, N-substituted maleimides, α-cyanocinnamic acid, esters of α-cyanocinnamic acid and an aliphatic alcohol which contains from 1 to 12 carbon atoms, and esters of fumaric acid and an aliphatic alcohol which contains from 1 to 12 carbon atoms. The copolymerization reaction that is described in the Japanese publication typically is conducted in the presence of a free radical polymerization initiator, such as acetyl peroxide, benzoyl peroxide, 2,2′-azobiscyclopropylpropionitrile, or the like. Such publication does not disclose or suggest the use of the described copolymers for photoresist compositions.
Typically, acrylates polymerize in the presence of radical or anionic initiators, whereas norbornenes do not readily follow radical pathways or respond to such anionic initiators. Rather, norbornene-type monomers are typically polymerized by cationic or insertion mechanisms. Therefore, in order to affect the copolymerization of acrylate-type monomers with norbornene-type monomers, it would be desirable to develop processes that would be effective for polymerizing both types of monomers. In addition, it would be desirable to formulate photo-imageable compositions that incorporate acrylate-norbornene polymers formed using any of the available processes for forming such polymers to take advantage of the properties of each monomer.